The ability to identify the geographic position of a user equipment (UE) in a wireless communications system has enabled and/or enhanced a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g., in the United States, where the Federal Communications Commission imposes regulatory requirements for Enhanced-911 services.
In many environments, the position of a UE can be accurately estimated by using positioning methods based on GPS (Global Positioning System). However, GPS is known to often fail in indoor environments and urban canyons. In these and other situations, the wireless communication system itself can assist the UE to determine its position with GPS. This approach is commonly referred to as Assisted-GPS positioning, or simply A-GPS, and serves to improve the UE receiver sensitivity and GPS start-up performance. Despite the possibility of this assistance, GPS and A-GPS still prove insufficient under some circumstances. Indeed, some UE's may not even be capable of using GPS or A-GPS.
A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by the 3rd Generation Partnership Project (3GPP). In addition to OTDOA, the Long Term Evolution (LTE) standard also specifies methods, procedures, and signalling support for Enhanced Cell ID (E-CID) and Assisted Global Navigation Satellite System (A-GNSS). Uplink Time Difference of Arrival (UTDOA) is also being standardized for LTE.
Positioning in LTE
The three key network elements in an LTE positioning architecture are the Location Services (LCS) Client, the LCS target device (i.e., the UE), and the LCS Server. The LCS Server estimates the position of the LCS target device. Specifically, the LCS Server is a physical or logical entity that manages positioning for the LCS target device by collecting measurements and other location information, that assists the LCS target device in measurements when necessary, and that estimates the LCS target device's position. The LCS Client may or may not reside in the LCS target device itself. Regardless, the LCS Client is a software and/or hardware entity that interacts with the LCS Server for the purpose of obtaining location information for the LCS target device. Specifically, the LCS Client sends a request to the LCS Server to obtain location information. The LCS Server processes and serves the received requests, and then sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the LCS target device or the network.
Position calculation can be conducted, for example, by a UE or by a positioning server, such as an Evolved Serving Mobile Location Center (E-SMLC) or Secure User Plan Location (SUPL) Location Platform (SLP) in LTE. The former approach corresponds to the UE-based positioning mode, whilst the latter corresponds to the UE-assisted positioning mode.
Two positioning protocols operating via the radio network exist in LTE, LTE Positioning Protocol (LPP) and LPP Annex (LPPa). The LPP is a point-to-point protocol between an LCS Server and an LCS target device, and is used in order to position the LCS target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel in order to reduce latency. LPPa is a protocol between an eNodeB and the LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. SUPL protocol is used as a transport for LPP in the user plane. LPP has also a possibility to convey LPP extension messages inside LPP messages, e.g. currently Open Mobiel Alliance (OMA) LPP extensions are being specified (LPPe) to allow e.g. for operator-specific assistance data, assistance data that cannot be provided with LPP, or to support other position reporting formats or new positioning methods.
A high-level architecture of such an LTE system 10 is illustrated in FIG. 1. In FIG. 1, the system 10 includes a UE 12, a radio access network (RAN) 14, and a core network 16. The UE 12 comprises the LCS target. The core network 16 includes an E-SMLC 18 and/or an SLP 20, either of which may comprise the LCS Server. The control plane positioning protocols with the E-SMLC 14 as the terminating point include LPP, LPPa, and LCS-AP. The user plane positioning protocols with the SLP 16 as the terminating point include SUPL/LPP and SUPL. Although note shown, the SLP 20 may comprise two components, a SUPL Positioning Center (SPC) and a SUPL Location Center (SLC), which may also reside in different nodes. In an example implementation, the SPC has a proprietary interface with E-SMLC, and an LIp interface with the SLC. The SLC part of the SLP communicates with a P-GW (PDN-Gateway) 22 and an External LCS Client 24.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons 26 is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
Positioning Methods
To meet Location Based Service (LBS) demands, the LTE network will deploy a range of complementing methods characterized by different performance in different environments. Depending on where the measurements are conducted and where the final position is calculated, the methods can be UE-based, UE-assisted, or network-based. Each of these approaches has its own advantages and disadvantages. The following methods are available in the LTE standard for both the control plane and the user plane: (1) Cell ID (CID), (2) UE-assisted and network-based E-CID, including network-based angle of arrival (AoA), (3) UE-based and UE-assisted A-GNSS (including A-GPS), and (4) UE-assisted OTDOA.
Hybrid positioning, fingerprinting positioning and adaptive E-CID (AECID) do not require additional standardization and are therefore also possible with LTE. Furthermore, there may also be UE-based versions of the methods above, e.g. UE-based GNSS (e.g. GPS) or UE-based OTDOA, etc. There may also be some alternative positioning methods such as proximity based location.
Similar methods, which may have different names, also exist in other RATs, e.g. WCDMA or GSM.
E-CID Positioning
E-CID positioning exploits the advantages of low-complexity and fast positioning associated with CID, but enhances positioning further with more measurement types. Specifically, CID exploits the network's knowledge of geographical areas associated with cell IDs. E-CID additionally exploits the corresponding geographical description of the serving cell, the Timing Advance (TA) of the serving cell, and the CIDs and the corresponding signal measurements of the cells (up to 32 cells in LTE, including the serving cell), as well as AoA measurements. The following UE measurements can be utilized for E-CID in LTE: E-UTRA carrier Received Signal Strength Indicator (RSSI), Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and UE Rx−Tx time difference. The E-UTRAN measurements available for E-CID are eNodeB Rx−Tx time difference (also called TA Type 2), TA Type 1 being (eNodeB Rx−Tx time difference)+(UE Rx−Tx time difference), and UL AoA, UE Rx−Tx measurements are typically used for the serving cell, whilst e.g. RSRP and RSRQ as well AoA can be utilized for any cell and can also be conducted on a frequency different from that of the serving cell.
The UE's E-CID measurements are reported by the UE to the positioning server (e.g. E-SMLC or SLP) over LPP, and the E-UTRAN E-CID measurements are reported by the eNodeB to the positioning node over LPPa. The UE may receive assistance data from the network e.g. via LPPe (no LPP assistance for E-CID is currently specified in the standard, however, it may be sent via LPP extension protocol, LPPe).
OTDOA Positioning
The OTDOA positioning method makes use of the measured timing of downlink signals received from multiple eNodeBs at the UE. The UE measures the timing of the received signals using assistance data received from the LCS server, and the resulting measurements are used to locate the UE in relation to the neighbouring eNodeBs.
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each (measured) neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between neighbor cell and the reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed.
To enable positioning in LTE, and to facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning have been introduced. 3GPP TS 36.211. These new signals are called positioning reference signals (PRS). Also, low-interference positioning subframes have been specified.
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern. 3GPP TS 36.211. A frequency shift, which is a function of Physical Cell Identity (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns and modelling the effective frequency reuse of six. This makes it possible to significantly reduce neighbour cell interference on the measured PRS and thus improve positioning measurements.
Assistance Data for Positioning
Assistance data is intended to assist a wireless device or a radio node in its positioning measurements. Different sets of assistance data are typically used for different methods. The positioning assistance data is typically sent by the positioning server, although it may be sent via other nodes. For example, assistance data may be sent to an eNodeB for being further sent to the UE, e.g. transparently to eNodeB and also Mobility Management Entity (MME). The assistance data may also be sent by the eNodeB via LPPa to positioning server for further transfer to the UE.
The assistance data may be sent responsive to a request from the wireless device that will perform measurements. Alternatively, the assistance data may be sent in an unsolicited way; that is, without request.
In LTE, the assistance data may be requested and provided over LPP protocol by including requestAssistanceData and provideAssistanceData elements in the LPP message, respectively. The current LTE standard specifies the structure shown in FIG. 11 for provideAssistanceData. In this structure, the commonIEsProvideAssistanceData IE is provided for future extensibility only, and is thus not currently used. The LTE assistance data may thus be provided for A-GNSS and OTDOA. The EPDU-Sequence contains IEs that are defined externally to LPP by other organizations, which currently may only be used for OMA LPP extensions (LPPe).
A similar structure exists for requestAssistanceData, and is shown in FIG. 12. In FIG. 12, commonIEsRequestAssistanceData may optionally carry the serving cell ID (ECGI).
OTDOA Assistance Data
Since for OTDOA positioning PRS signals from multiple distinct locations need to be measured, the UE receiver may have to deal with PRS that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to do signal search within a large window. Such a search would impact the time and accuracy of the measurements, as well as the UE complexity. To facilitate UE measurements, the network transmits assistance data to the UE, which includes, among others things, reference cell information, a neighbour cell list containing PCIs of neighbour cells, the number of consecutive downlink subframes, PRS transmission bandwidth, frequency, etc.
For OTDOA, the assistance data is provided with IE OTDOA-ProvideAssistanceData which comprises the information about the reference cell (one cell in the list) and neighbor cells information (multiple cells). This IE is shown in FIG. 13.
The neighbour cells may or may not be on the same frequency as the reference cell, and the reference cell may or may not be on the same frequency as the serving cell. Measurements that involve cells on a frequency different than the serving cell are inter-frequency measurements. Measurements on the same frequency as the serving cell are intra-frequency measurements. Different requirements apply for intra- and inter-frequency measurements.
The current standard only allows including E-UTRA cells in the assistance data. However, the cells may still belong to FDD and TDD which are treated as different RATs.
E-CID Assistance Data
Assistance data delivery is not required for UE- or eNodeB-assisted forms of E-CID positioning. In fact, this is not currently supported without EPDU elements. Also, UE-based E-CID location is not currently supported either, and the assistance data delivery procedure is not applicable to uplink E-CID positioning. No assistance data is currently specified for E-CID for LPP. Some assistance data, however, may be provided for E-CID e.g. via LPPe.
Assistance Data Extensions with OMA
With Open Mobile Alliance (OMA) LPP extension (LPPe), the assistance data is enhanced with the possibility to assist a larger range of positioning methods (e.g. assistance data may also be provided for E-CID or other methods of other RATs, e.g. OTDOA UTRA or E-OTD GSM, or other PLMN networks). Furthermore, there is also a possibility of carrying over a black-box data container meant for carrying vendor-/operator-specific assistance data.
Inter-Frequency, Inter-Band and Inter-RAT Measurements
It is mandatory for all UEs to support all intra-RAT measurements (i.e. inter-frequency and intra-band measurements) and meet the associated requirements. However the inter-band and inter-RAT measurements are UE capabilities, which are reported to the network during call setup. The UE supporting certain inter-RAT measurements should meet the corresponding requirements. For example a UE supporting LTE and WCDMA should support intra-LTE measurements, intra-WCDMA measurements and inter-RAT measurements (i.e. measuring WCDMA when the serving cell is LTE and measuring LTE when the serving cell is WCDMA). Hence the network can use these capabilities according to its strategy. These capabilities are highly driven by factors such as market demand, cost, typical network deployment scenarios, frequency allocation, etc.
Inter-Frequency Measurements
Inter-frequency measurements may in principle be considered for any positioning method, even though currently not all measurements are specified by the standard as intra- and inter-frequency measurements. When performing inter-frequency measurement, the serving and target carrier frequencies may belong to the same duplex mode or to different duplex modes e.g. LTE FDD-FDD inter-frequency, LTE TDD-TDD inter-frequency, LTE FDD-TDD inter-frequency or LTE TDD-FDD inter-frequency scenario. The FDD carrier may operate in full duplex or even in half duplex mode. The examples of inter-frequency measurements currently specified by the standard are Reference Signal Time Difference (RSTD) used for OTDOA, RSRP and RSRQ which may be used e.g. for fingerprinting or E-CID.
The UE performs inter-frequency and inter-RAT measurements in measurement gaps. The measurements may be done for various purposes: mobility, positioning, self organizing network (SON), minimization of drive tests etc. Furthermore the same gap pattern is used for all types of inter-frequency and inter-RAT measurements. Therefore E-UTRAN must provide a single measurement gap pattern with constant gap duration for concurrent monitoring (i.e. cell detection and measurements) of all frequency layers and RATs.
In LTE, measurement gaps are configured by the network to enable measurements on the other LTE frequencies and/or other RATs (e.g. UTRA, GSM, CDMA2000, etc). The gap configuration is signaled to the UE from the serving cell radio node over the Radio Resource Control (RRC) protocol as part of the measurement configuration. A UE that requires measurement gaps for positioning measurements, e.g., OTDOA, may send an indication to the network, e.g. eNodeB, upon which the network may configure the measurement gaps. Furthermore, the measurement gaps may need to be configured according to a certain rule, e.g. inter-frequency RSTD measurements for OTDOA require that the measurement gaps are configured according to the inter-frequency requirements in 36.133, Section 8.1.2.6, e.g. not overlapping with PRS occasions of the serving cell and using gap pattern #0.
In a carrier aggregation system, there may be multiple serving cells. In this case, a set of serving cells for a UE in a carrier aggregation mode comprises one primary cell and one or more configured secondary cells. A carrier aggregation capable UE generally does not require measurement gaps for performing measurements on configured and activated primary and secondary cells. However, there may be cells in the system that are not configured or not activated as serving cells for the UE, e.g., for one of the following reasons: the UE may be capable of supporting only a limited number of serving cells and/or some cells may be deactivated for carrier aggregation or not configured as secondary cells. For performing measurements on these cells, the UE would normally still require measurement gaps.
Inter-RAT Measurements
In general, in LTE inter-RAT measurements are typically defined similar to inter-frequency measurements. That is, inter-RAT measurements may also require configuring measurement gaps, but just with more measurement restrictions and often more relaxed requirements. As a special example there may also be multiple networks, which use overlapping sets of RATs. The examples of inter-RAT measurements specified currently for LTE are UTRA FDD CPICH RSCP, UTRA FDD carrier RSSI, UTRA FDD CPICH Ec/No, GSM carrier RSSI, and CDMA2000 1x RTT Pilot Strength.
For positioning, assuming that LTE FDD and LTE TDD are treated as different RATs, the current standard defines inter-RAT requirements only for FDD-TDD and TDD-FDD measurements, and the requirements are different in the two cases. There are no other inter-RAT measurements specified within any separate RAT for the purpose of positioning and which are possible to report to the positioning node (e.g. E-SMLC in LTE).
Inter-Band Measurements
Inter-band measurement refers to the measurement done by the UE on a target cell on the carrier frequency belonging to a frequency band different than that of the serving cell. Both inter-frequency and inter-RAT measurements can be intra-band or inter-band.
The motivation of inter-band measurements is that most of the UEs today support multiple bands even for the same technology. This is driven by the interest from service providers; a single service provider may own carriers in different bands and would like to make efficient use of carriers by performing load balancing on different carriers. A well known example is that of multi-band GSM terminal with 800/900/1800/1900 bands. Another example is when a DL band has no paired UL within the same band and thus has to be paired with UL from another frequency band.
Furthermore a UE may also support multiple technologies e.g. GSM, UTRA FDD and E-UTRAN FDD. Since all UTRA and E-UTRA bands are common, therefore the multi-RAT UE may support same bands for all the supported RATs.
Inter-Frequency Requirements for Positioning-Related Timing Measurements
No inter-frequency requirements are currently defined for UE or eNodeB Rx−Tx measurements. For OTDOA, the current standard defines inter-frequency requirements for RSTD measurements assuming the following two scenarios, 3GPP TS 36.133. In the first scenario, the reference cell and all neighbor cells provided in the assistance data operate on the same frequency f2, which is different from the serving cell frequency f1. In the second scenario, the reference cell is on the serving cell frequency f1, whilst all neighbor cells provided in the assistance data are on frequency f2, which is different from the serving cell f1. The requirements are generic with respect to the frequency channels and frequency bands, i.e. the requirements are the same for any two different f1 and f2, independently on their absolute and relative location in the spectrum. In real deployments there may also be intermediate scenarios between the first scenario and the second scenario. Further, although the requirements are only defined for two frequencies, the signalling specified for OTDOA positioning supports up to three frequencies that may be different from the reference cell frequency, which in turn may also be different from the serving/primary cell frequency.
Problems with Existing Solutions
At least the following problems have been identified with the prior art solutions.
An eNodeB is heretofore unable to properly configure measurement gaps for a UE performing inter-frequency RSTD measurements. Indeed, the eNodeB is not even aware of the frequency or cell IDs on which the measurements are to be performed and thus is not be able e.g. to align PRS positioning occasions with measurement gaps. The consequence is that the measurement gaps are incorrectly configured, or do not provide sufficiently many or a required number of subframes with PRS for positioning measurements. This means that the UE measurements may fail or the measurement requirements may not be met.
Furthermore, there is currently no way to configure and use measurement gaps for inter-RAT positioning, for the measurements requested via assistance data received using LPPe or the user plane, or for non PRS-based measurements (which may or may not be performed according to a pattern, e.g., a restricted measurement pattern configured with enhanced inter-cell interference coordination (eICIC)).
Moreover, the measurement gaps to be applied by the UE are configured by the eNodeB over RRC. However it is the positioning server, e.g. E-SMLC, which is aware of whether and when the UE will conduct positioning inter-frequency measurements such as e.g., inter-frequency RSTD or inter-frequency E-CID and this information is transmitted to the UE transparently via the eNodeB. Thus, to be on the safe side the eNodeB may always configure UEs for the worst case, i.e. with Gap Pattern #0 (where a measurement gap of 6 ms occurs every 40 ms), even when the UEs measure only on intra-frequency cells. This is a severe restriction on the network in that it reduces the amount of radio resources available for intra-frequency measurements and it leads to an inefficient measurement procedure.